The First Noel is perhaps three hundred years old:
Some of the phrasing is hard to understand because it uses older grammar that is now no longer used. Indeed, I suspect that some of the grammar was old-fashioned even when it was written. Some of it sounds like someone in 1800 trying to write like a person from 1650. Some of the phrasing is contorted just because it’s an attempt to fit words into a pre-existing melody. Let’s go through some of the song:
The first Noel the angels did say
Was to certain poor shepherds in fields as they lay;
In fields where they lay, keeping their sheep,
On a cold winter’s night that was so deep.
What this means is this:
The first Christmas carol is the one that angels sang to some poor shepherds in the fields where they slept, taking care of their sheep, on a winter’s night that was deep.
What I’m not sure of is what the phrase “night that was so deep” means. I think that I had the idea when I heard the song as a child was that it meant that the snow was deep that night. I don’t think it makes much sense for there to be deep snow in Israel. However, the songwriters might not have known this. The phrase may simply mean that it was in the middle of the night.